The structures, energetics, spectroscopies, and isomerization of possible low-lying Si2P2 isomers in both singlet and triplet states are theoretically investigated at the B3LYP/6-311G(d) and CCSD(T)/6-311+G(2df) (single-point) levels. At the final CCSD(T)/6-311+G(2df)//B3LYP/6-311G(d) level, the lowest energy isomer is a singlet butterfly-like SiPSiP structure (1)1 with P-P cross bonding followed by a singlet rhombic SiPSiP isomer (1)2 with Si-Si cross bonding, whereas the cyanogen analogue PSiSiP (1)5 is the highest lying of all the singlet isomers. The singlet potential energy surface of Si2P2 indicates that the rhombic isomer (1)2 is kinetically much more stable than the butterfly-like isomer (1)1, although isomer (1)2 is 3.2 kcal/mol higher in energy than isomer (1)1, while other isomers are kinetically unstable toward isomerization to isomer (1)1 or (1)2. It is also shown that the triplet Si2P2 isomers are energetically higher than all the single species except (1)5. Furthermore, for the most relevant singlet Si2P2 isomers and interconversion transition states, the relative energies obtained at the B3LYP/6-311G(d) level are in excellent agreement with the values calculated at the single-point CCSD(T)/6-311+G(2dF) level within 2 kcal/mol. Finally, the structural, energetic and kinetic similarities and discrepancies between the isomers of Si2P2 and other analogous molecules C2N2, Si2N2 and C2P2 are compared and analyzed.